An important aspect of all forms of low dosage forms is related to the stability of the same. The stability of a pharmaceutical dosage form is related to maintaining its physical, chemical, microbiological, therapeutic, and toxicological properties when stored, i.e., in a particular container and environment. Stability study requirements are covered, e.g., in the Good Manufacturing Practices (GMPs), the U.S.P., as well as in New Drug Applications (NDAs) and Investigational New Drug Applications (INDs).
The ingredients used in sustained release dosage formulations often present special problems with regard to their physical stability during storage. For example, waxes which have been used in such formulations are known to undergo physical alterations on prolonged standing, thus precautions are taken to stabilize them at the time of manufacture or to prevent the change from occurring. Fats and waxy materials when used in purified states are known to crystallize in unstable forms, causing unpredictable variations in availability rates during stability testing at the time of manufacture and during later storage.
It is known that certain strategies can be undertaken to obtain stabilized controlled release formulations in many cases, such as insuring that the individual ingredients are in a stable form before they are incorporated into the product, and that processing does not change this condition, retarding the instability by including additional additives, and inducing the individual ingredients of the dosage form to reach a stable state before the product is finally completed.
It is also recognized that the moisture content of the product can also influence the stability of the product. Changes in the hydration level of a polymeric film, such as the ethyl celluloses, can alter the rate of water permeation and drug availability. Also, binders such as acacia are known to become less soluble when exposed to moisture and heat. However, moisture content of a product can be controlled fairly successfully by controls in the processing method and proper packaging of the product.
Hydrophobic polymers such as certain cellulose derivatives, zein, acrylic resins, waxes, higher aliphatic alcohols, and polylactic and polyglycolic acids have been used in the prior art to develop controlled release dosage forms. Methods of using these polymers to develop controlled release dosage forms such as tablets, capsules, suppositories, spheroids, beads or microspheres are to overcoat the individual dosage units with these hydrophobic polymers. It is known in the prior art that these hydrophobic coatings can be applied either from a solution, suspension or dry. Since most of these polymers have a low solubility in water, they are usually applied by dissolving the polymer in an organic solvent and spraying the solution onto the individual drug forms (such as beads or tablets) and evaporating off the solvent.
Aqueous dispersions of hydrophobic polymers have been used in the prior art to coat pharmaceutical dosage forms for aesthetic reasons such as film coating tablets or beads or for taste-masking. However, these dosage forms are used for immediate release administration of the active drug contained in the dosage form.
Attempts to prepare stable controlled release pharmaceutical formulations using aqueous dispersions of hydrophobic polymers have been unsuccessful due to stability problems.
Therefore, it is desirable to prepare a controlled release formulation prepared from an aqueous dispersion of a hydrophobic polymer. However, to date, attempts to prepare stable controlled release pharmaceutical formulations using aqueous dispersions of hydrophobic polymers have been unsuccessful due to stability problems.
In particular, when coating these pharmaceutical forms using aqueous polymeric dispersions to obtain a desired release profile of the active drug(s) over several hours or longer, it is known in the art that the dissolution release profile changes on ageing. This was recently demonstrated by Munday, et al., Drug Devel. and Indus. Phar., 17 (15) 2135-2143 (1991), which reported the effect of storing theophylline mini-tablets film coated with ethyl cellulose with PEG (2:1 ratio; total coating=3% w/w), ethyl cellulose with Eudragit.RTM. L (2:1 ratio; total coating=3% w/w); and Eudragit.RTM. RL (amount of coating=1.5% w/w) at varying temperatures and relative humidities upon the rate of drug release. Samples were subjected to isothermal storage at 28.degree. C., 35.degree. C. and 45.degree. C. with the relative humidity (RH) maintained between 55-60%, under cyclic conditions of 45.degree. C. at 55% RH for 24 hours, then at 28.degree. C. and 20% RH for 24 hours, and then at 5.degree. C. and 10% RH for 24 hours, after which the cycle was repeated, and alternating conditions every 24 hours between 45.degree. C. and 55% RH and 28.degree. C. and 0% RH. The aging process brought about by storage under the above stress conditions impeded dissolution, irrespective of the nature of the polymeric film. The greatest reduction in release rate was said to occur in the first 21 days (isothermal storage) after coating.
This instability problem is known to not exist when the polymers are applied from organic solvent solution. The use of organic solvents in the preparation of polymer coatings is considered problematic as the formulations have inherent problems with regard to flammability, carcinogenicity, environmental concerns, and safety in general.
Furthermore, attempts to prepare controlled release pharmaceutical formulations using organic coatings have been largely unsuccessful due to stability problems, the rate of drug release being changed upon storage.
For example, it has been considered desirable in the art to prepare a controlled release formulation which utilizes a retardant coating derived from an aqueous acrylic polymer dispersion, such as Eudragit.RTM., commercially available from Rohm Pharma. However, to date it has not been possible to obtain a controlled release formulation which is stable under various storage conditions.
More particularly, it is known that a controlled release coating comprising Eudragit.RTM. is not stable when cured according to recommended curing conditions by the manufacturer of 45.degree. C. for 2 hours.